GB2054195A

GB2054195A - Colour Selective Directional Mirrors

Info

Publication number: GB2054195A
Application number: GB8022749A
Authority: GB
Original assignee: AGA AB
Current assignee: AGA AB
Priority date: 1979-07-11
Filing date: 1980-07-11
Publication date: 1981-02-11
Also published as: DE3026370A1; JPS5635109A

Abstract

A colour selective directional mirror esp. for a head-up display is formed to selectively reflect one or more beams of incident radiation in a selected direction or respective directions, the mirror comprising a reflective interference filter layer 1 and at least one absorption filter layer 2, 3 to assist in defining the colour band selection. The layer 1 may be a multi- layer interference filter or a hologram. The absorption and interference layers may be of non-uniform thickness to compensate for different angles of incidence. As shown, the mirror has a transparent substrate 61. <IMAGE>

Description

SPECIFICATION Colour Selective Directional Mirrors The present invention relates to colour selective directional mirrors selecting radiation of one or more wavelengths incident within a restricted angular field of view to be reflected in at least one defined direction. Thus, only selected radiation which lies within one or a limited number of narrowly defined wavelength bands and which may enter as a convergent or divergent beam or beams from one or a limited number of directions.

Such reflective devices are sometimes referred to as "dark mirrors", since the only radiation to be reflected is that which comes from one or more predetermined directions and which lies within one or more predetermined, narrow limited wavelength ranges. The designation "dark mirror" relates in this context to the circumstance that those portions of the spectrum which the mirror is not intended to reflect are suppressed to the greatest possible extent. Thus portions of the mirror surface frequently appear dark to the observer or instrument. "Dark mirrors" composed of one or more selective narrow-band reflective interference filter layers have the disadvantage of being sensitive to the angle of incidence of the radiation, the reflected wavelength being displaced towards shorter wavelengths as the angle of incidence increases.This wavelength displacement is a function of the cosine of the angle of incidence within a stack of interference filter layers. The consequence is that "dark mirrors" built up in this manner are limited to use with a restricted range of angles of incidence, and for substantially parallel beams of incident radiation. This implies that there will be difficulties in fields of use in which incident radiation arrives from several different directions, and especially if it is desired to have the mirror function as a colour selective filter when using a convergent or divergent incident beam or beams, since it may then be difficult to obtain the required degree of narrow limited wavelength bands.This is due to the fact that the reflection of the interference filter must be made effective over a broad-band in order to accomodate the entire angular range of the incident radiation, so that radiation at wavelengths outside the desired selectively chosen wavelength range but incident within the same angular field of view will also be reflected by the mirror.

The invention consists in a colour selective directional mirror in which only selected radiation within a sharply defined band of the spectrum is reflected in a selected direction, said mirror comprising a reflective filter layer operating by virtue of optical interference phenomena, and being provided with at least one absorption filter in the path of incident radiation.

The reflective coating may be a grating, a multilayer reflective interference filter or a reflective holographic layer. By placing the reflective layer or layers on one face of an optical element of material transparent to the selected radiation, such as glass, plastic, silicon or the like, and placing one or more of the relatively incidence-insensitive absorption filters on the opposite surface of the element, a narrow limitation to the desired wavelength bands can be accompished. Moreover, if the optical element is wedge-shaped, then with the reflective coating on the back of the element, non-desired reflections in the boundary layers between absorption layers and optical elements can be directed away in directions other than the direction in which the selected light is intended to be reflected.

A mirror of this type is desirable where it is necessary to reflect a limited wavelength band from an object which may, for example, be a display screen. In particular, the mirror may be designed for use in aircraft in order to introduce into the field of vision of the aircraft pilot symbols displayed on one or more screens. In such arrangements, problems are encountered in excluding solar radiation from the screen and preventing other radiation from the surroundings from entering the optical system of the head-up type display and causing reflections and reduction of contrast, which can be extremely disturbing. If several screens are used, it may be necessary to have the symbols on all of these reflected onto one and the same image plane.

If the mirror is to be inserted in a convergent or divergent beam of radiation and the reflecting surface comprises a reflective interference filter, then it is possible to make allowance for different angles of incidence of the radiation on different sections of the interference layer by making the interference layer so that it has different reflection properties on different parts of the surface. In addition, it is possible to take into consideration the attenuation of the different layers in relation to the direction of incidence and reflection of the radiation and consequently of its path through the respective layers and the attenuation which imparts to the radiation within an intended wavelength range and also with regard to the reflection characteristic curve of the respective layer or layers used in that the thickness of the absorption layer varies along the surface.

In a simple embodiment, the mirror comprises a plate with a reflective interference filter layer or a reflective holographic layer on a front surface, and one or a plurality of absorption filter layers superimposed on top. If the mirror is to be inserted in the path of a convergent or divergent beam obliquely incident, then the absorption layers are preferably wedge-shaped.

The invention will now be described with reference to the drawings, in which four exemplary embodiments are schematically illustrated respectively in Figures 1 to 4.

Figure 1 shows a first exemplary embodiment of a mirror according to the invention. This comprises a reflective holographic or interference filter layer 1 with a reflective curve which, for the actual wavelength range to be reflected has a directionally selective reflection range which comprises an angle of incidence range of approximately +5 around the main direction of the incident radiation. This implies that the colour reflectance range of the mirror coating must be significantly broader than the actual wavelength range for the mirror. Moreover, it is quite impossible to make a reflective interference or holographic filter which does not reflect at all outside the intended colour reflectance range.

There is always some weak residual reflectance over substantially the entire spectrum.

For this reason, the reflective filter layer 1 is applied to the rear of an absorption filter, which as shown in the figure can comprise a combination filter composed of separate absorption filters, in this case layers 2 and 3, which are glued to each other with a glue layer 4, which may itself be coloured to serve as an absorption filter layer. The absorption filter combination has a transmission interval which is narrowly defined around the actual wavelength range which the mirror is to reflect. An antireflection layer 5 of conventional type is provided on the entry surface.

A reflective interference or holographic filter layer coating is used due to the fact that it is very difficult to obtain a sufficiently narrow limited wavelength band by using absorption filters only.

The reflectance curve of the mirror coating and the transmission curve of the absorption filters are so adapted in relation to each other that one limiting wavelength of the mirror coating, preferably the longest, is located close to one limiting wavelength of the transmission range of the absorption filter so that the reflectance range of the mirror is limited to the range between these two limiting wavelengths. Due to the character of a reflective interference or holographic filter, it is difficult to avoid certain minor reflection peaks entering within the residual portion of the transmission range of the absorption filter. The reflective coating is dimensioned to have the smallest possible reflection peaks within this range.If possible, this range may also be located beyond the spectral sensitivity range of the human eye, or of any radiation detector which may be used, or at least close to the limit of the sensitivity range, where sensitivity is decreasing.

Both the reflective coating and the absorption filter are dimensioned to have high slope of the reflectance and transmission characteristic, respectively, at the limit ranges for the desired reflectance range for the mirror.

If the radiation incident on the mirror is collimated, the direction of incidence will be the same over the entire mirror surface and consequently, the mirror layer 1 can then be so composed that it has equal properties over its entire surface. This means that if the mirror uses a reflective interference filter with various dielectric layers incorporated, these each have the same respective thickness over the entire mirror surface.

If, in contrast, the incident radiation is a divergent or convergent beam, the angle of incidence (or in practice the local range of angles of incidence) will be different on different parts of the surface.

The dielectric layers of a reflective interference filter will then be made with different thicknesses at different parts of the mirror surface, in order to adapt the reflection curve of the mirror to the various angles of incidence. For example, if the angle of incidence is 350 +30 at one edge of the mirror and 458 +30 at the other edge of the mirror, the reflection curve would be displaced towards lower wavelengths by approximately 0.1 6R, where A is the wavelength to be reflected, unless the thicknesses of the dielectrical layers included in the mirror layer were adapted in the manner described.

Reflective interference layers with varying thickness over their surface may be obtained by vacuum deposition with careful orientation of the surface to be coated in relation to the source and/or the use of appropriately designed masks between the surface and the source.

As shown in broken lines, at the back of the reflective layer 1 is a layer 61 which is significantly thicker than any of the other layers.

This layer 61 is necessary as a mechanical support unless a mirror formed only by absorption layers and the reflective filter coating were found to be completely mechanically stable. The layer 61 consists of a rigid material, such as glass or plastic with a smooth surface. Where a carrier 61 is found to be necessary, the reflective interference layer 1 can be appropriately vaporized onto the surface of the latter, and the absorption filter 2 then glued on. If the carrier 61 consists of glass, its bottom side may be matt, or may have a non-transparent surface layer.

Figure 2 shows a second embodiment in which a reflective filter layer 1 is arranged on the back of a wedge-shaped optical element 6 of material transparent to the light used such as optical glass, silicon or the like. On the front of the element 6 are two slightly wedge-shaped absorption filters, 7 and 8, glued in place by glue layers 9 and 10.

One of these absorption filters may be a low-pass filter with a steep edge at the limiting wavelength and the other filter can then be a band-pass filter which is selected to have a steep edge at the upper frequency limit of the intended wavelength range.

The sequence between these filters is of no importance, provided that the filters otherwise have similar properties. For example, if one filter should change its transmission properties under the influence of incident ultraviolet radiation, and the other does not allow such radiation to pass, it is obvious that theqfirst mentioned of these filters is to be placed behind the second.

The mirror is shown in the path of a divergent beam of radiation which enters towards the mirror from the left, where one side of the beam has an angle of incidence of 1 and the other of a. In the example shown, the different components 6 to 10 have substantially the same refractive index, and therefore the beam direction is only changed by refraction upon entry into and exit from the composite mirror. This is naturally not an essential condition for execution of the invention, but if the absorption layers, in order to perform their controlling function, must be given a different shape than a pure wedge-shape, this is appropriate.The incident radiation is reflected by the reflective layer 1 if the selected wavelength, and passes back through units 6-10 and the components 6 to 10 to leave at angles of Q:3 and a, respectively.

It is evident from the figure that beams with different angles of incidence will have paths of different lengths through the absorption layers if these have completely plane parallel surfaces, and this would give different levels of attenuation to the transmitted radiation at different portions of the mirror. In order to avoid this, the absorption layers are made with varying thickness over the surface so that a uniform attenuation of the entire beam can be obtained. It may naturally be difficult to achieve more complicated thickness variation in the two absorption layers, since this imposes exacting demands on grinding of the respective surfaces of the absorption layer and the optical element.Therefore a good approximation for a solution of this problem is to employ wedgeshaped absorption layers arranged with the tip of the wedge disposed in the plane of incidence of the entering beam and with the tip turned away from the direction from which the radiation approaches. In Figure 2, the wedge tip of the absorption filters is directed towards the right.

Broken arrows 11 and 12 in Figure 2 show that radiation reflected from the front of the optical element (as well as the radiation reflected against the different filter layers and the upper edge of element 6) is reflected in a direction entirely different to that of the selected radiation, which is reflected by the reflective filter layer and is to be utilised after having passed through other optical elements (not shown).

A broken line in Figure 2 shows an extra absorption layer 1 3 with a glue layer 14, between the filter 7 and the element 6. It is obvious that the reflective layer 1 can be made so that it has reflection bands for several wavelength ranges.

This may, for example, be appropriate if the mirror is to be inserted to reflect pictures drawn on a colour cathode ray tube when the respective colours red, blue and green of which the picture on the screen is composed are selectively filtered out by the mirror. This gives an extremely sharply drawn picture.The mirror built up with the extra filter 13 shown in Figure 2 is adapted to reflect two colours, and filter 7, for example, may be a low-pass filter with a sharp transmission edge at the lower frequency limiting wavelength of the lower range, filter 8 may be a pass-band filter for the lower frequency wavelength range and a further pass-band for the upper range, or a lowpass filter for defining the upper wavelength range, and filter 13 may have a pass-band range which extends over both the actual wavelength ranges or else may have two or more pass-bands, where two of these extend over each and everyone of the wavelength ranges for which the mirror is adapted.

The reflective filter coating 1 has reflectance bands for the actual wavelength ranges which are made as narrow as possible in that only a part of each reflectance band of interest lies within a transmission range for the absorption filter combination.

A full specification for one exemplary embodiment of the type shown in Figure 2, in which reflection is wanted forA= 543 ,am with angles of incidence from 150 to 450 with a largely uni-dimensional variation across the surface with a mean angle of incidence at one side of approximately 200 and at the other side of approximately 360 and in each point with a reflected angular range of approximately 150 for the actual wavelength. Layer 8 consists of BG367 0 and layer 7 consists of OG53016 where the designation of glass types and thicknesses is made in accordance with Schott's glass catalogue.The reflective interference layer 1 consists of 25 layers, alternately of TiO2 having a refractive index approximately equal to 2.40, and of SiO2 having a refractive index approximately equal to 1.45, the sequence of the layers being.

(H/2) LHLH...HL (H/2), where L represents an SiO2 layer, which at the centre has a thickness of 875 A; H is a TiO2 with a thickness of 518 ; and H/2 is a TiO2 layer with a thickness of 259 .

At one side with the larger mean angle of incidence, the thickness of each layer is increased by approximately 4% and at the other side with the smaller angle of incidence, the thickness of each layer is reduced by approximately 2%.

Figure 3 shows yet another exemplary embodiment, designed to reflect selected radiation coming from two different radiation sources, with mutually different or similar wavelength ranges, and to pass them out in the same direction to enter a common optical system (not shown) placed after the mirror in the direction of the radiation path. Here, as in the embodiment according to Figure 2, a wedgeshaped element 1 6 of material transparent to the intended light, such as glass, if the mirror is intended for light within the visible range, is provided on its lower side, as drawn, with a reflective interference filter layer 17. On the other side of the optical element, an absorption layer 18 is secured by a glue layer 19.Above this layer, in this embodiment, there are two further absorption layers, 20 and 21, the layer 20 having provided on its lower side an auxiliary reflective interference filter layer 22. The components 20 and 21 are secured by glue layers 23 and 24. The combination of components 1 6 to 22 reflects radiation coming from a source that is to the left in the figure and whose path is shown by broken lines, whilst the layers 20, 21 and 22 are provided to reflect the radiation whose beam is shown by chain-dotted lines, and which comes from a radiation source (not shown) that is to the right, as drawn. The emergent beam paths for the radiation from the two radiation sources are to be substantially similar after reflection from the mirror. The radiation sources do not need to lie in the plane of the paper, and one may be above this plane and the other below.

In order for this combining effect to be obtainable, layers 20, 21 and 22 have high transmittance for the wavelength range of the radiation from the right-hand source in order for this radiation not to be attenuated to any appreciable extent. Since absorption layer 1 8 is adapted for the right-hand radiation source, the tip of its wedge is directed towards the left, away from this source. The two absorption layers 20 and 21 are adapted for the radiation coming from the left-hand radiation source.These may both be band-pass filters with their essential pass band around the radiation emanating from the righthand source but with weaker pass-bands around other wavelength ranges, these having been chosen in this case to lie around the wavelength range for the left-hand radiation source and the filters having been made sufficiently thin to act as band-pass filters for the radiation from the lefthand source. The various angles of incidence for the radiation towards the mirror have been so chosen that any radiation of the wavelength which is transmitted through the absorption layer 18, and which comes from the left-hand source does not lie within the selective direction of incidence range that is reflected by the layer 17.

Figure 4 shows yet another exemplary embodiment of the invention, in which a reflective holographic layer 25 is disposed on the back of a stack of absorption filters 26, 27 and 28, secured to one another by glue layers 29 and 30. With a reflective holographic layer 25 normal defining the direction of reflection may be different to the line normal to the actual layer 25. In thick-film holograms, from which reflective holographic filter components are usually made, several different holograms may be superimposed on each other in the same film. This implies that a holographic mirror can be manufactured so that one layer may reflect incident radiation in different directions or, as in the case shown in Figure 4, may reflect separate beams of radiation which are directed towards the layer from two different directions to emerge in the same direction.The mirror shown in Figure 4 is shown inserted in convergent beam paths from two radiation sources, one beam of radiation being marked by chain-dotted lines, from the left of the other, the radiation path of which is marked by broken lines. The two radiation sources are monochromatic and may have the same or different wavelengths, as determined by the hologram construction. Both beam paths are reflected by the holographic mirror in the direction shown by the beam path drawn with an unbroken line. As in the case of reflective interference filters, reflective holographic filters are highly sensitive to changes in the wavelength of the entering light and, more specifically are such that a wavelength alteration gives a change in the direction of reflection for the entering radiation. If the mirror is intended to be able to reflect radiation of two different colours coming from two different predetermined respective directions, then the absorption filter layer stack 26, 27 and 28, must provide for transmission of the required colours. The or each of the holograms may be utilised to form required display characters or symbols, and any of the embodiments shown in Figures 1 to 3 may utilise a reflective holographic filter layer.

Claims

1. A colour selective directional mirror in which only selected radiation within a sharply defined band of the spectrum is reflected in a selected direction, said mirror comprising a reflective filter layer operating by virtue of optical interference phenomena, and being provided with at least one absorption filter in the path of incident radiation.

2. A mirror as claimed in Claim 1, in which said reflective filter layer is a multi-layer interference filter.

3. A mirror as claimed in Claim 1, in which said reflective filter layer is a reflective hologram or a layer forming a composite plurality of holograms.

4. A mirror as claimed in any preceding Claim, in which there is provided a plurality of absorption filters.

5. A mirror as claimed in any preceding Claim, in which said reflective filter layer is mounted on a carrier plate and has said absorption filter or filters superimposed thereon.

6. A mirror as claimed in any one of Claims 1 to 4, in which said reflective filter layer is mounted on one surface of an optical element that carries said absorption filter or filters on its opposite surface.

7. A mirror as claimed in Claim 6, in which said optical element is wedge-shaped.

8. A mirror as Claimed in any preceding Claim, in which said reflective filter is such that separate incident beams are reflected in a common output direction.

9. A mirror as claimed in any one of Claims 1 to 7, in which said reflective filter is such that separate incident beams are reflected in respective selected directions.

10. A mirror as claimed in Claim 8 or Claim 9, in which said separate incident beams are of the same wavelength.

1 A mirror as claimed in Claim 8 or Claim 9, in which said separate incident beams are of mutually different wavelengths.

12. A mirror as claimed in any preceding Claim, in which said incident beam or each of said beams is convergent.

13. A mirror as claimed in any one of Claims 1 to 11, in which said incident beam or each of said beams is divergent.

14. A mirror as claimed in Claim 2, or any one of Claims 4 to 13 when dependent upon Claim 2, in which said interference filter layer is graded to compensate for differing angles of incidence.

1 5. A mirror as claimed in any preceding Claim, in which the or each said absorption filter is graded to compensate for differing angles of incidence.

1 6. A mirror as claimed in any preceding Claim, in which an auxiliary reflective interference filter is positioned in the path of incident radiation to said reflective filter layer.

1 7. A colour selective directional mirror substantially as described with reference to any one of Figures 1 to 4.